Effects of Combined Acid-alkali and Heat Treatment on the Physiochemical Structure of Moso Bamboo
酸碱联合热处理对毛竹理化结构的影响
To improve the performance of bamboo and increase its utilization value, this study aimed at investigating the effects of impregnation pretreatment and thermal treatment on the structural changes of bamboo. The samples were pretreated in sodium hydroxide or zinc chloride solution, and then treated at 160 °C. The pretreated and control samples were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG), and Fourier transform infrared spectroscopy (FTIR). The results showed that the cellulose crystallinity and intensity of samples pretreated by ZnCl2 could be reduced, but the crystal structure remained the same. As for samples pretreated in NaOH, the crystal structure of fiber was destroyed and the crystallinity was increased significantly. High temperature treatment has little effect on the thermal stability of bamboo. However, after treatment with NaOH and ZnCl2, the thermal degradation temperature changed obviously and moved to a lower temperature. ZnCl2 pretreatment had influence on the chemical structure of bamboo, while NaOH pretreatment had greater influence on the chemical structure of bamboo.
为了提高竹材的性能和利用价值，本研究旨在探讨浸渍预处理和热处理对竹材结构变化的影响。样品在氢氧化钠或氯化锌溶液中预处理，然后在160°C下处理。用X射线衍射(XRD)、热重分析(TG)和傅里叶变换红外光谱(FTIR)对预处理和对照样品进行了表征。结果表明，ZnCl₂预处理样品的纤维素结晶度和强度减少，但晶体结构保持不变。对于在NaOH中预处理的样品，纤维的晶体结构被破坏，结晶度显著提高。高温处理对竹材的热稳定性影响不大。 然而，经过NaOH和ZnCl₂的处理热降解温度变化明显，移动到较低的温度。ZnCl₂预处理对竹子的化学结构有一定影响，而NaOH预处理对竹子的化学结构有较大的影响。
Bamboo, which is characterized by abundant reserves, fast growth, high strength and rigidity, good processability, and strong local availability, is an important sustainable and renewable non-wood forest resource in China. At present, bamboo is widely applied in the production of various kinds of bamboo-based composite materials, such as bamboo plywood, laminated bamboo lumber, oriented strand board, and bamboo plastic composite materials, and these products are all widely applied as building materials. However, bamboo, as one of the lignocellulose materials composed of cellulose, hemicellulose and lignin, has many shortcomings. For example, bamboo contains high contents of sugars, starch, and proteins, and is thus susceptible to attack by various molds under high humidity conditions. Like wood and other biological materials, bamboo undergoes (shrinkage and swelling) hygroexpansion, and would change its dimensions with the variation of moisture content in its service conditions. As a result, defects such as shrinkage, cracking, and transformation can occur, thus reducing its value.
Thermal modification is environmentally friendly and has been widely applied to decrease the hygroscopicity of wood, improve the dimensional stability, durability, weathering ability, antibacterial performance and other characteristics. Additionally, the existing research also indicates that heat treatment can decrease the moisture absorption group content and improve the dimensional stability of bamboo. Moreover, afer heat treatment at 160 °C, 180 °C, and 200 °C, the thickness expansion and anti-corrosion performance of Cizhu recombined bamboo can be improved, but the strength performance is decreased. However, due to the complex structure of bamboo, the existing bamboo heat treatment is time consuming, and contributes to environmental pollution due to the high content of saccharides, such as sugars and starch, which generate smoke during the heat treatment process.
Additionally, acid/alkali pretreatment is also widely used in the field of bamboo. Alkali pretreatment can effectively delignify cellulose, chemically expand cellulose and enzymatically saccharify bamboo, and cut of the chemical connection between hemicellulose and lignin, removing most of lignin and hemicellulose. Acid treatment can promote hydrolysis of bamboo lignocellulose, increase metal absorption and reduce organic adsorption. It has been reported by many researchers that acid/alkali pretreatment has certain effects on crystallinity, chemical structure and thermal stability of bamboo fibers. Li et al. used 1% NaOH to study bamboo fiber, the results showed that the surface morphology, crystallinity and chemical elements of bamboo fiber had been changed after alkali pretreatment. Lin et al. applied NaOH solutions with different concentrations (4%, 6%, and 8%), and concluded that alkali treatment could dissolve impurities, wax, hemicellulose and lignin of bamboo fiber, which could improve crystallinity but not thermal stability. There are also some studies on the combination of acid/alkali treatment and low temperature heat treatment. At two different temperatures of 117 °C and 135 °C, three pretreatment methods of sodium fber, sulfuric acid and glycerol are used to investigate the chemical composition and structural characteristics of pretreated bamboo fibers changed, and it is concluded that NaOH pretreatment achieved the best enzyme digestibility at higher temperatures. In addition, studies have shown that when the concentration of NaOH is greater than 12%, the cellulose I start to transform to cellulose II, which is more stable. Sugiman et al. studied the effects of 4, 8 and 12% NaOH on crystallinity and chemical composition of bamboo fibers, and reached a consistent conclusion.
Although there are many studies on bamboo heat treatment, the problems of high energy consumption in actual heat treatment process were still need to be solved, thus, the combined acid-alkali was applied to modify moso bamboo to decrease its heat treatment decomposition temperature in this study. Additionally, relatively low concentration acid/alkali pretreatment is studied in terms of changing the crystallinity, chemical structure and thermal stability of bamboo fibers, while rare works had focus on the effects of high-concentration alkali pretreatment on bamboo fiber structure and thermal stability, which could change the physiochemical structure of moso bamboo significantly, and improve bamboo characteristics combining with heat treatment. In this paper, the acid and alkali pretreatment has been applied to solve the problems of high energy consumption in actual heat treatment process. 15% sodium hydroxide and ZnCl2 were used to pretreat the specimens respectively, and these specimens were further treated with heat treatment at 160 °C. All the samples were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG), and Fourier transform infrared spectroscopy (FTIR) to analysis the cellulose crystallinity and intensity, thermal decomposed characteristics and chemical structure.
竹子具有储量丰富、生长速度快、强度和刚性高、加工性好、本地可利用性强等特点，是我国重要的可持续可再生非木材森林资源。目前，竹材广泛应用于各种竹基复合材料的生产，如竹胶合板、层压竹材、定向链板和竹塑复合材料，这些产品都被广泛应用于建筑材料中。然而，竹子作为由纤维素、半纤维素和木质素组成的木质纤维素材料之一，存在许多缺点。例如，竹子含有高含量的糖、淀粉和蛋白质，因此在高湿度条件下容易受到各种霉菌的攻击。与木材和其他生物材料一样，竹子会经历吸湿膨胀，并会随着其使用条件中水分含量的变化而改变其尺寸。因此，可发生收缩、开裂和转变等缺陷，从而降低其价值。热改性对环境友好，已广泛应用于降低木材的吸湿性，提高木材的尺寸稳定性、耐久性、耐候性、抗菌性能等特点。此外，现有的研究还表明，热处理可以降低竹材的吸湿基团含量，提高竹材的尺寸稳定性。此外，在160°C、180°C和200°C下进行热处理后，可以提高慈竹重组竹的厚度膨胀和防腐性能，但强度性能下降。然而，由于竹子结构复杂，现有的竹子热处理耗时，由于糖和淀粉等糖类含量高，在热处理过程中产生烟雾，导致环境污染。此外，酸/碱预处理也被广泛应用于竹子的堆焊。碱预处理可有效地去除纤维素，化学膨胀纤维素和酶法糖
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